Feeder fiber and central office redundancy

ABSTRACT

A carrier office includes an optical line terminal, a first transmit-erbium-doped fiber amplifier (EDFA), and a second transmit-EDFA. The OLT is configured to transmit first and second optical signals. The first transmit-EDFA is optically coupled to the OLT and a first feeder fiber, and the first feeder fiber is optically coupled to a first remote node (RN). The first transmit-EDFA is operable between a respective enabled state and a respective disabled state. The second transmit-EDFA is optically coupled to the OLT and a second feeder fiber, and the second feeder fiber is optically coupled to a second RN. The second transmit-EDFA is operable between a respective enabled state and a respective disabled state.

TECHNICAL FIELD

This disclosure relates to providing feeder fiber and central officeredundancy in time-wavelength division multiplexed (TWDM) passiveoptical networks (PONs).

BACKGROUND

Fiber optic communication is an emerging method of transmittinginformation from a source (transmitter) to a destination (receiver)using optical fibers as the communication channel. WDM-PON is an opticaltechnology for access and backhaul networks. WDM-PON uses multipledifferent wavelengths over a physical point-to-multipoint fiberinfrastructure that contains passive optical components. The use ofdifferent wavelengths allows for traffic separation within the samephysical fiber. The result is a network that provides logicalpoint-to-point connections over a physical point-to-multipoint networktopology. WDM-PON allows operators to deliver high bandwidth to multipleendpoints over long distances. A PON generally includes an optical lineterminal located at a service provider central office (e.g., a hub), aremote node connected to the central office by a feeder fiber, and anumber of optical network units or optical network terminals, near endusers. The remote node demultiplexes an optical signal from the centraloffice and distributes the demultiplexed optical signals to multipleoptical network terminals along corresponding distribution fibers.Time-division-multiplexing (TDM) is a method of transmitting andreceiving independent signals over a common signal path by usingdifferent, non-overlapping time slots. Time wavelength divisionmultiplexing (TWDM) uses both time and wavelength dimensions tomultiplex signals.

The reliability of communications networks is generally very important.Core and metro sections of most networks typically include rings ofredundant paths to prevent service outages due to fiber cuts and siteoutages. The redundant rings serve a large number of customers, allowingthe additional costs of implementing redundancy to be shared by a largenumber of users.

SUMMARY

One aspect of the disclosure provides a carrier office including anoptical line terminal (OLT), a first transmit-erbium-doped fiberamplifier (EDFA), and a second transmit-EDFA. The OLT is configured totransmit first and second optical signals. The first transmit-EDFA isoptically coupled to the OLT and a first feeder fiber. Moreover, thefirst feeder fiber is optically coupled to a first remote node (RN). Thefirst transmit-EDFA is operable between a respective enabled state and arespective disabled state. The enabled state of the first transmit-EDFAis configured to allow the first optical signal transmitted from the OLTto pass through the first transmit-EDFA to the first RN. The disabledstate of the first transmit-EDFA is configured to substantially inhibitthe passing of the first optical signal from the OLT through the firsttransmit-EDFA to the first RN. The second transmit-EDFA is opticallycoupled to the OLT and a second feeder fiber. The second feeder fiber isoptically coupled to a second RN. The second transmit-EDFA is operablebetween a respective enabled state and a respective disabled state. Theenabled state of the second transmit-EDFA is configured to allow thesecond optical signal transmitted from the OLT to pass through thesecond transmit-EDFA to the second RN. The disabled state of the secondtransmit-EDFA is configured to substantially inhibit the passing of thesecond optical signal from the OLT through the second transmit-EDFA tothe second RN.

Implementations of the disclosure may include one or more of thefollowing optional features. In some implementations, the OLT includes alaser array configured to transmit multiple optical signals. The OLT mayalso include one or more transceivers each configured to transmit acorresponding downstream optical signal at a corresponding targetwavelength.

In some examples, the carrier office includes a multiplexer opticallycoupled to the OLT and a splitter optically coupled to the multiplexerand the first and second transmit-EDFAs. The multiplexer is configuredto multiplex the first and second optical signals into a multiplexedsignal. The splitter may be configured to split the multiplexed signalreceived from the multiplexer into first and second split-multiplexedsignals, route the first split-multiplexed signal to the firsttransmit-EDFA, and route the second split-multiplexed signal to thesecond transmit-EDFA. The carrier office may also include: i) a firstband multiplexer optically coupled to the first transmit-EDFA and thefirst feeder fiber; and ii) a second band multiplexer optically coupledto the second transmit-EDFA and the second feeder fiber. The first bandmultiplexer configured to receive the first split-multiplexed signalfrom the first transmit-EDFA and pass the first split-multiplexed signalas a downstream component of the first optical signal to the first RN.The second band multiplexer configured to receive the secondsplit-multiplexed signal from the second transmit-EDFA and pass thesecond split-multiplexed signal as a downstream component of the secondoptical signal to the second RN.

In some implementations, the carrier office includes a controller incommunication with the first and second transmit-EDFAs. The controllermay be configured to receive a remote node status indicating whether thesecond RN is receiving optical signals from another carrier office, andwhen the remote node status indicates that the second RN is notreceiving optical signals from the other carrier office, instruct thesecond transmit-EDFA to be in the enabled state. The controller may alsobe configured to, when the remote node status indicates that the secondRN is receiving optical signals from the other carrier office, instructthe second transmit-EDFA to be in the disabled state.

In some examples, the carrier office includes a first receive-EDFAoptically coupled to the OLT and the first feeder fiber and a secondreceive-EDFA optically coupled to the OLT and the second feeder fiber.The first receive-EDFA may be operable between a respective enabledstate and a respective disabled state. The enabled state of the firstreceive-EDFA may be configured to allow a third optical signal from thefirst RN to pass through the first receive-EDFA to the OLT. The disabledstate of the first receive-EDFA may be configured to substantiallyinhibit the passing of the third optical signal from the first RNthrough the first receive-EDFA to the OLT. Similarly, the secondreceive-EDFA may be operable between a respective enabled state and arespective disabled state. The enabled state of the second receive-EDFAmay be configured to allow a fourth optical signal from the second RN topass through the second receive-EDFA to the OLT. The disabled state ofthe second receive-EDFA may be configured to substantially inhibit thepassing of the fourth optical signal from the second RN through thesecond receive-EDFA to the OLT.

In some examples, the carrier office includes: i) a first bandmultiplexer optically coupled to the first receive-EDFA and the firstfeeder fiber; and ii) a second band multiplexer optically coupled to thesecond receive-EDFA and the second feeder fiber. The first bandmultiplexer may be configured to demultiplex out the third opticalsignal from the first optical signal and pass the third optical signalto the first receive-EDFA. The second band multiplexer may be configuredto demultiplex out the fourth optical signal from the second opticalsignal and pass the fourth optical signal to the second receive-EDFA.The carrier office may also include a combiner optically coupled to thefirst and second receive-EDFAs and a demultiplexer optically coupled tothe combiner and the OLT. The combiner may be configured to receive thethird and fourth optical signals from the first and second receive-EDFAsand combine the third and fourth optical signals into acombined-multiplexed signal. The demultiplexer may be configured todemultiplex the combined-multiplexed signal into first and seconddemultiplexed optical signals and route the first and seconddemultiplexed optical signals to the OLT.

Another aspect of the disclosure provides an optical network including afirst carrier office (CO) configured to transmit a first optical signaland a second CO configured to transmit a second optical signal, thesecond optical signal being the same as the first optical signal. Theoptical network also includes a first feeder fiber optically coupled tothe first CO, a second feeder fiber optically coupled to the second CO,a RN optically coupled to the first feeder fiber and the second feederfiber, and a controller in communication with the first and second COs.The controller is configured to perform operations including instructingthe first CO to transmit the first optical signal to the RN along thefirst feeder fiber and determining whether the RN receives the firstoptical signal. When the RN fails to receive the first optical signal,the controller is configured to instruct the second CO to transmit thesecond optical signal to the RN along with the second feeder fiber.

This aspect may include one or more of the following optional features.In some implementations, the first CO includes a first OLT configured totransmit the first optical signal and a first transmit-EDFA opticallycoupled to the first OLT and the first feeder fiber. The firsttransmit-EDFA is operable between a respective enabled state and arespective disabled state. The enabled state of the first transmit-EDFAmay be configured to allow the first optical signal transmitted from thefirst OLT to pass through the first transmit-EDFA to the RN. Thedisabled state of the first transmit-EDFA may be configured tosubstantially inhibit the passing of the first optical signal from thefirst OLT through the first transmit-EDFA to the RN. The second COincludes a second OLT configured to transmit the second optical signaland a second transmit-EDFA optically coupled to the OLT and the secondfeeder fiber. The second transmit-EDFA may be operable between arespective enabled state and a respective disabled state. The enabledstate of the second transmit-EDFA may be configured to allow the secondoptical signal transmitted from the second OLT to pass through thesecond transmit-EDFA to the RN. The disabled state of the secondtransmit-EDFA may be configured to substantially inhibit the passing ofthe second optical signal from the second OLT through the secondtransmit-EDFA to the RN.

In some examples, the operations include receiving a remote node statusindicating whether the RN is receiving first optical signal from thefirst CO, and when the remote node status indicates that the RN is notreceiving first optical signals from the first CO, instructing thesecond transmit-EDFA to be in the enabled state. The operations mayfurther include, when the remote node status indicates that the RN isreceiving the first optical signal from the first CO, instructing thesecond transmit-EDFA to be in the disabled state.

Yet another aspect of the disclosure provides a method for providingcentral office redundancy by allowing multiple central offices toservice a given remote node. The method includes transmitting a firstoptical signal from a first CO along a first feeder fiber to a first RN,receiving a remote node status indicating whether the second RN isreceiving optical signals from a second CO, and when the remote nodestatus indicates that the second RN is not receiving optical signalsfrom the second CO, instructing the second transmit-EDFA to be in theenabled state. The first CO includes an OLT, a first transmit-EDFAoptically coupled to the OLT and the first feeder fiber, and a secondtransmit-EDFA optically coupled to the OLT and a second feeder fiber.The OLT is configured to transmit the first optical signal and a secondoptical signal. The first feeder fiber is optically coupled to the firstRN. Moreover, the first transmit-EDFA is operable between a respectiveenabled state and a respective disabled state. The enabled state of thefirst transmit-EDFA is configured to allow the first optical signaltransmitted from the OLT to pass through the first transmit-EDFA to thefirst RN. The disabled state of the first transmit-EDFA is configured tosubstantially inhibit the passing of the first optical signal from theOLT through the first transmit-EDFA to the first RN. The second feederfiber is optically coupled to a second RN. The second transmit-EDFAoperable between a respective enabled state and a respective disabledstate. The enabled state of the second transmit-EDFA is configured toallow the second optical signal transmitted from the OLT to pass throughthe second transmit-EDFA to the second RN. The disabled state of thesecond transmit-EDFA is configured to substantially inhibit the passingof the second optical signal from the OLT through the secondtransmit-EDFA to the second RN.

This aspect may include one or more of the following optional features.In some implementations, when the remote node indicates that the secondRN is receiving optical signals from the second CO, the method includesinstructing the second transmit-EDFA to be in the disable state. The OLTmay include a laser array configured to transmit multiple opticalsignals. The OLT may also include one or more transceivers eachconfigured to transmit a corresponding downstream optical signal at acorresponding target wavelength.

In some examples, the first CO includes a multiplexer optically coupledto the OLT and configured to multiplex the first and second opticalsignals into a multiplexed signal, a splitter optically coupled to themultiplexer and the first and second transmit-EDFAs, a first bandmultiplexer optically coupled to the first transmit-EDFA and the firstfeeder fiber, and a second band multiplexer optically coupled to thesecond transmit-EDFA. The splitter may be configured to split themultiplexed signal received from the multiplexer into first and secondsplit-multiplexed signals, route the first split-multiplexed signal tothe first transmit-EDFA, and route the second split-multiplexed signalto the second transmit-EDFA. The first band multiplexer may beconfigured to receive the first split-multiplexed signal from the firsttransmit-EDFA and pass the first split-multiplexed signal as adownstream component of the first optical signal to the first RN. Thesecond band multiplexer may be configured to receive the secondsplit-multiplexed signal from the second transmit-EDFA and pass thesecond split-multiplexed signal as a downstream component of the secondoptical signal to the second RN.

In some implementations, the method includes receiving, at the first CO,a third optical signal from the first RN along the first feeder fiber.The first CO may also include a first receive-EDFA optically coupled tothe OLT and the first feeder fiber and a first receive-EDFA opticallycoupled to the OLT and the first feeder fiber. The first receive-EDFAmay be operable between a respective enabled state and a respectivedisabled state. The enabled state of the first receive-EDFA may beconfigured to allow a third optical signal from the first RN to passthrough the first receive-EDFA to the OLT. The disabled state of thefirst receive-EDFA may be configured to substantially inhibit thepassing of the third optical signal from the first RN through the firstreceive-EDFA to the OLT. Similarly, the second receive-EDFA may beoperable between a respective enabled state and a respective disabledstate. The enabled state of the second receive-EDFA may be configured toallow a fourth optical signal from the second RN to pass through thesecond receive-EDFA to the OLT. The disabled state of the secondreceive-EDFA may be configured to substantially inhibit the passing ofthe fourth optical signal from the second RN through the secondreceive-EDFA to the OLT.

The first CO may further include a first band multiplexer opticallycoupled to the first receive-EDFA and the first feeder fiber and asecond band multiplexer optically coupled to the second receive-EDFA andthe second feeder fiber. The first band multiplexer may be configured todemultiplex out the third optical signal from the first optical signaland pass the third optical signal to the first receive-EDFA. The secondband multiplexer may be configured to demultiplex out the fourth opticalsignal from the second optical signal and pass the fourth optical signalto the second receive-EDFA. In some examples, the first CO includes acombiner optically coupled to the first and second receive-EDFAs and ademultiplexer optically coupled to the combiner and the OLT. Thecombiner may be configured to receive the third and fourth opticalsignals from the first and second receive-EDFAs and combine the thirdand fourth optical signals into a combined-multiplexed signal. Thedemultiplexer may be configured to demultiplex the combined-multiplexedsignal into first and second demultiplexed optical signals and route thefirst and second demultiplexed optical signals to the OLT.

Yet another aspect of the disclosure provides a second method forproviding central office redundancy by allowing multiple central officesto service a given remote node. The method includes transmitting, froman OLT of a first CO, a first optical signal, amplifying, by a firsttransmit-EDFA of the first CO, the first optical signal, sending theamplified first optical signal from the first CO along a first feederfiber to a first remote node (RN), and receiving a remote node statusindicating whether a second RN is receiving optical signals from asecond CO. When the remote node status indicates that the second RN isnot receiving optical signals from the second CO, the method includestransmitting, from the OLT of the first CO, a second optical signal,amplifying, by a second transmit-EDFA of the first CO, the secondoptical signal, and sending the amplified second optical signal from thefirst CO along a second feeder fiber to the second RN.

This aspect may include one or more of the following optional features.In some implementations, when the remote node status indicates that thesecond RN is receiving optical signals from the second CO, the methodincludes at least one of ceasing transmission of the second opticalsignal from the OLT or ceasing amplification of the second opticalsignal by the second transmit-EDFA. The cessation of amplification ofthe second optical signal may cause the second transmit-EDFA tosubstantially inhibit the passing of the second optical signal throughthe second transmit-EDFA to the first RN. The OLT may include a laserarray configured to transmit multiple optical signals. The OLT may alsoinclude one or more transceivers each configured to transmit acorresponding downstream optical signal at a corresponding targetwavelength.

In some examples, the method includes multiplexing, by a multiplexeroptically coupled to OLT, the first and second optical signals into amultiplexed signal, splitting, by a splitter optically coupled to themultiplexer and the first and second transmit-EDFAs, the multiplexedsignal into first and second split-multiplexed signals, routing, by thesplitter, the first split-multiplexed signal to the first transmit-EDFA,and routing, by the splitter, the second split-multiplexed signal to thesecond transmit-EDFA. The method may also include allowing, by a firstband multiplexer optically coupled to the first transmit-EDFA and thefirst feeder fiber, transmission of the first split-multiplexed signalas a downstream component of the first optical signal to the first RNand allowing, by a second band multiplexer optically coupled to thesecond transmit-EDFA and the second feeder fiber, transmission of thesecond split-multiplexed signal as a downstream component of the secondoptical signal to the second RN. In some examples, the method includesreceiving, at a first receive-EDFA optically coupled to the first feederfiber, a third optical signal from the first RN, amplifying, by thefirst receive-EDFA, the third optical signal, and routing the thirdoptical signal from the first receive-EDFA to the OLT.

When the remote node status indicates that the second RN is receivingoptical signals from the second CO, the method may include disablingamplification by a second receive-EDFA of a fourth optical signalreceived from the second RN along the second feeder fiber. The disablingof amplification by the second receive-EDFA may substantially inhibitreceipt of the fourth optical signal through the second receive-EDFA.When the remote node status indicates that the second RN is notreceiving optical signals from the second CO, the method may includeenabling amplification by the second receive-EDFA of the fourth opticalsignal received from the second RN along the second feeder fiber androuting the fourth optical signal from the second receive-EDFA to theOLT.

In some implementations, the method includes allowing, by a first bandmultiplexer optically coupled to the first receive-EDFA and the firstfeeder fiber, the third optical signal to pass from the first RN to thefirst receive-EDFA. The method may also include allowing, by a secondband multiplexer optically coupled to the second receive-EDFA and thesecond feeder fiber, the fourth optical signal to pass from the secondRN to the second receive-EDFA. The method may further include receiving,at a combiner optically coupled to the first and second receive-EDFAs,the third and fourth optical signals from the first and secondreceive-EDFAs and combining, by the combiner, the third and fourthoptical signals into a combined-multiplexed signal. The method may alsoinclude receiving, at a demultiplexer optically coupled to the splitterand the OLT, the combined-multiplexed signal, demultiplexing, by thedemultiplexer, the combined-multiplexed signal into first and seconddemultiplexed optical signals, and routing, by the demultiplexer, thefirst and second demultiplexed optical signals to the OLT.

The details of one or more implementations of the disclosure are setforth in the accompanying drawings and the description below. Otheraspects, features, and advantages will be apparent from the descriptionand drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of an example communication system.

FIGS. 2A and 2B are schematic views of example arrayed waveguidegratings.

FIG. 3 illustrates an example time-wavelength division multiplexedarchitecture for a communication system.

FIGS. 4A-4C are schematic views of a communication system offeringfeeder fiber redundancy for at least one remote node to alleviate asingle central office from being a point of failure for the at least oneremote node.

FIG. 5A is a schematic view of an example optical signal processor of aremote node optically coupled to two different central offices.

FIG. 5B is a schematic view of an example wavelength divisionmultiplexer of a remote node.

FIG. 5C is a schematic view of an example remote node including anoptical combiner combining two optical signals into a combined opticalsignal.

FIG. 5D is a schematic view of an example two-by-two optical switch of aremote node optically coupled to two different central offices.

FIGS. 6A and 6B are schematic views of the two-by-two optical switch ofFIG. 5D.

FIG. 7 is a schematic view of operational stages of an exampletwo-by-two optical switch during failure and restoration of a centraloffice or a corresponding feeder fiber optically coupled to the opticalswitch.

FIG. 8 is a schematic view of an example arrangement of operations for amethod of providing central office redundancy by allowing multiplecentral offices to service a given remote node.

FIG. 9 is a schematic view of an example arrangement of operations for amethod of switching a two-by-two optical switch at a remote node.

FIG. 10 is schematic view of an example computing device that may beused to implement the systems and methods described in this document.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Referring to FIG. 1, a communication system 100 delivers communicationsignals 102 (e.g., optical signals) through communication links 110, 110a-n, 112, 112 a-n (e.g., optical fibers or line-of-sight free spaceoptical communications) between an optical line terminal (OLT) 120housed in a central office (CO) 130 and optical network units (ONUs)140, 140 a-n (e.g., a bidirectional optical transceiver) associated withusers 150, 150 a-n (also referred to as customers or subscribers). TheONUs 140, 140 a-n are typically located at premises 152, 152 a-n of theusers 150, 150 a-n.

Customer premises equipment (CPE) is any terminal and associatedequipment located at the premises 152 of the user 150 and connected to acarrier telecommunication channel C at a demarcation point (“demarc”).In the examples shown, the ONU 140 is a CPE. The demarc is a pointestablished in a house, building, or complex to separate customerequipment from service provider equipment. CPE generally refers todevices such as telephones, routers, switches, residential gateways(RG), set-top boxes, fixed mobile convergence products, home networkingadapters, or Internet access gateways that enable the user 150 to accessservices of a communications service provider and distribute them aroundthe premises 152 of the user 150 via a local area network (LAN).

In some implementations, the optical communication system 100 implementsan optical access network 105, such as a passive optical network (PON)105, for example, for access and mobile fronthaul/backhaul networks. Insome examples, the optical communication system 100 implements apoint-to-point (pt-2-pt) PON having direct connections, such as opticalEthernets, where a home-run optical link 110, 112 (e.g., fiber) extendsall the way back to an OLT 120 at the CO 130 and each customer 150, 150a-n is terminated by a separate OLT 120 a-n. In other examples, theoptical communication system 100 implements a point-to-multi-point(pt-2-multi-pt) PON, where a shared OLT 120 services multiple customers150, 150 a-n.

The CO 130 includes at least one OLT 120 connecting the optical accessnetwork 105 to an Internet Protocol (IP), Asynchronous Transfer Mode(ATM), or Synchronous Optical Networking (SONET) backbone, for example.Therefore, each OLT 120 is an endpoint of the PON 105 and convertsbetween electrical signals used by service provider equipment andoptical signals 102 used by the PON 105. Each OLT 120, 120 a-n includesat least one transceiver 122, 122 a-n, depending on the implementationof the optical access network 105. The OLT 120 sends the optical signal102 via a corresponding transceiver 122, through a feeder fiber 110 to aremote node (RN) 170, which demultiplexes the optical signal 102 anddistributes the demulitplexed optical signals 102 to multiple users 150,150 a-n along corresponding distribution fibers 112, 112 a-n. A centralcontroller 450 may monitor the operational state of one or more COs 130,and thereby determine whether a default CO 130 is servicing an RN 170.In some examples, the central controller 450 instructs a CO 130 to offerfeeder fiber redundancy to a given RN 170, that by default, is servicedby another CO 130 that has failed or a feeder fiber cut preventscommunications between the RN 170 and the default CO 130. Themultiplexer 160 for multiplexing/demultiplexing may be an arrayedwavelength grating 200 (AWG), which is a passive optical device. In someexamples, each CO 130 includes multiple OLTs 120, 120 a-n, and each OLT120 is configured to service a group of users 150. In addition, each OLT120 may be configured to provide signals in different services, e.g.,one OLT 120 may provide services in 1G-PON, while another OLT 120provides services in 10G-PON.

As shown in FIG. 1, the CO 130 multiplexes signals received from severalsources, such as a video media distribution source 132, an Internet datasource 134, and a voice data source 136, and multiplexes the receivedsignals into one multiplexed signal 102 before sending the multiplexedoptical signal 102 to the RN 170 through the feeder fiber 110. Themultiplexing may be performed by the OLT 120 or a broadband networkgateway (BNG) positioned at the CO 130. Typically, services aretime-division-multiplexed on the packet layer.

Time-division-multiplexing (TDM) is a method of transmitting andreceiving independent signals over a common signal path by usingdifferent, non-overlapping time slots. Wavelength division multiplexing(WDM) uses multiple wavelengths λ to implement point-to-multi-pointcommunications in the PON 105. The OLT 120 serves multiple wavelengthsthrough one fiber 110 to the multiplexer 160 at the RN 170, whichmultiplexes/demultiplexes signals between the OLT 120 and a plurality ofONUs 140, 140 a-n. Multiplexing combines several input signals andoutputs a combined signal. Time wavelength division multiplexing (TWDM)uses both time and wavelength dimensions to multiplex signals.

For WDM, the OLT 120 includes multiple optical transceivers 122, 122a-n. Each optical transceiver 122 transmits signals at one fixedwavelength λ_(D) (referred to as a downstream wavelength) and receivesoptical signals 102 at one fixed wavelength λ_(U) (referred to as anupstream wavelength). The downstream and upstream wavelengths λ_(D),λ_(U) may be the same or different. Moreover, a channel C may define apair of downstream and upstream wavelengths λ_(D), λ_(U), and eachoptical transceiver 122, 122-n of a corresponding OLT 120 may beassigned a unique channel C_(a-n).

The OLT 120 multiplexes/demultiplexes the channels C, C_(a-n) of itsoptical transceivers 122, 122 a-n for communication of an optical signal102 through the feeder fiber 110. Whereas, the multiplexer 160 at the RN170 multiplexes/demultiplexes optical signals 102, 104, 104-n betweenthe OLT 120 and a plurality of ONUs 140, 140 a-n. For example, fordownstream communications, the multiplexer 160 demultiplexes the opticalsignal 102 from the OLT 120 into ONU optical signals 104, 104-n, i.e.,downstream optical signals 104 d, for each corresponding ONU 140, 140a-n. For upstream communications, the multiplexer 160 multiplexes ONUoptical signals 104, 104-n from each corresponding ONU 140, 140 a-n,i.e., upstream optical signals 104 u, into the optical signal 102 fordelivery to the OLT 120. To make the transmission successful, theoptical transceivers 122, 122 a-n of the OLT 120 match with the ONUs140, 140-n one-by-one. In other words, the downstream and upstreamwavelengths λ_(D), λ_(U) (i.e., the channel C) of respective downstreamand upstream optical signals 104 d, 104 u to and from a given ONU 140matches the downstream and upstream wavelengths λ_(D), λ_(U) (i.e., thechannel C) of a corresponding optical transceiver 122. In someimplementations, each ONU 140, 140 a-n includes a corresponding tunableONU transceiver 142, 142 a-n (e.g., that includes a laser or lightemitting diode) that can tune to any wavelength λ used by acorresponding OLT 120 at a receiving end. The ONU 140 may automaticallytune the tunable ONU transceiver 142 to a wavelength λ that establishesa communication link between the corresponding OLT 120 and the ONU 140.Each optical transceiver 122, 142 may include data processing hardware124, 144 (e.g., circuitry, field programmable gate arrays (FPGAs), etc.)and memory hardware 126, 146 in communication with the data processinghardware 124, 144. The memory hardware 126, 146 may store instructions(e.g., via firmware) that when executed on the data processing hardware124, 144 cause the data processing hardware 124, 144 to performoperations for auto-tuning the optical transceiver 122, 142. The ONU 140may include a photodetector that converts the optical wave to anelectric form. The electrical signal may be further amplified and thende-multiplexed down to subcomponents (e.g., data over a network, soundwaves converted into currents using microphones and back to its originalphysical form using speakers, converting images converted into currentsusing video cameras and converting back to its physical form using atelevision). Additional details on auto-tuning the ONU 140 tocommunicate with the corresponding OLT 120 can be found in U.S. patentapplication Ser. No. 15/354,811, filed on Nov. 17, 2016, which is herebyincorporated by reference in its entirety.

FIGS. 2A and 2B illustrate an exemplary arrayed waveguide grating 200(AWG), which may be used as a multiplexer 160. An AWG 200 may be used todemultiplex an optical signal in a WDM system or a TWDM system. AWGs 200can multiplex a large number of wavelengths λ into one optical fiber,thus increasing the transmission capacity of optical networks. AWGs 200can therefore multiplex channels of several wavelengths λ onto a singleoptical fiber at a transmission end, and reciprocally they can alsodemultiplex different wavelength channels at the receiving end of anoptical access network 105. An AWG 200 is a passive planar light wavecircuit device typically used as a wavelength multiplexer and/ordemultiplexer. N×N AWGs 200 also have wavelength routing capabilities.If a system has N equally-spaced wavelengths λ_(n), an N×N AWG 200 canbe designed with an egress port spacing matching the wavelength spacing.The N×N AWG 200 routes differing wavelengths λ, λ_(1-n) at an ingressport 210, 210 a-n to different egress ports 220, 220 a-n such that all Nwavelengths λ_(1-n) are mapped to all N egress ports 220 a-nsequentially. The routing of the same N wavelengths λ_(1-n) at twoconsecutive ingress ports 210 have the wavelength mapping shifted by oneegress side.

The AWG 200 is cyclic in nature. The wavelength multiplexing anddemultiplexing property of the AWG 200 repeats over periods ofwavelengths called free spectral range (FSR). Multiple wavelengths,separated by the free spectral range (FSR), are passed down each port220. Therefore, by utilizing multiple FSR cycles, different tieredservices may coexist on the same fiber plant.

FIG. 3 illustrates an example TWDM architecture 300 for thecommunication system 100 that facilitates user aggregation onto a singlestrand of fiber 110, 110 a-n, 112, 112 a-n and PON reach. Multiple PONs105, 105 a-n are stacked using different wavelengths λ via thisarchitecture. The different PONs 105 are combined using wavelengthselective multiplexers 160, typically AWGs 200 or thin film filters(TFF). At the RN 170, a cyclical AWG 200 separates the differentwavelengths λ, sending each wavelength PON 105 to a different group ofusers 150. In some examples, the cyclical AWG 200 allow up to 20different wavelengths λ, thus giving the ability to stack up to 20 PONs105. Some wavelengths λ may be used for point-to-point services and somewavelengths λ may be used for point-to-multipoint TDM services, asshown. For TDM, one or multiple stages of splitters 180 can be usedafter the AWG 200 in order to split the optical signals 102 from thefeeder fiber 110 out to each single-wavelength optical signal 104, 104a-n of each PON 105 to each corresponding ONU 140, 140 a-n.Incorporating splitters 180 after the AWG 200 reduces the number of AWGs200 required and also allows point-to-point services to bypass thesplitters. One of the splitter stages 180 may be co-located with thecyclical AWG 200, but co-location is not required. In order to extendthe reach of each PON 105, erbium-doped fiber amplifiers (EDFAs) 430 mayamplify both downstream and upstream optical signals 104 d, 104 u insidethe CO 130. EDFAs 430 are typically directional, so the downstream andupstream optical signals 104 d, 104 u may be amplified separately.Additional details on example TWDM architectures are explained in U.S.patent application Ser. No. 14/952,321, filed on Nov. 25, 2015, which ishereby incorporated by reference in its entirety.

The CO 130 can be the largest single point of failure in the opticalcommunication system 100. To remedy this single point of failure, eachRN 170 may be connected to two different COs 130. Although not oftenrealized in deployment, most OLT vendors do have a solution to supportthis level of redundancy that uses two sets of OLTs 120 in each CO 130.However, having a full set of idle OLTs 120 adds a large upfront costand increases space and power requirements of the COs 130. Multiple setsof OLTs 120 may be co-located at a single CO 130 as old OLTs areupgraded with improved OLTs. For instance, one or more OLTs 120 mayprovide services in 1G-PON, while new OLTs 120 may be upgraded at agiven CO 130 to provide services in 10G-PON.

Referring to FIGS. 4A-4C, in some implementations, the communicationsystem 100 offers feeder fiber redundancy for both RNs 170, 170 a, 170 bto alleviate the possibility of a single CO 130 being a point of failurefor both RNs 170. The communication system 100 offers feeder fiberredundancy by having the RN 170 in communication with at least two COs130, 130 a, 130 b, so that if a first CO 130 a (e.g., CO1) orcorresponding first feeder fiber (FF1) 110 a fails, a second CO 130 b(e.g., CO2) can maintain communications for the RN 170. In the examplesshown, at the first CO 130 a, a first OLT 120 a corresponds to a defaultOLT 120 a for the first RN 170, 170 a (RN1) and a backup OLT 120 a forthe second RN 170, 170 b (RN2), while at the second CO 130 b, a secondOLT 120 b corresponds to a default OLT 120 b for the second RN 170 b(RN2) and a backup OLT 120 b for the first RN 170 a.

Each CO 130 a, 130 b includes one or more fiber expanders 400, 400 a,400 b to multiplex/demultiplex optical signals 102 a, 102 b, 102 a′, 102b′ from the OLT ports and provide separate amplification for the opticalsignals 102 a, 102 b, 102 a′, 102 b′ transmitted/received to and fromthe at least one RN 170, 170 a, 170 b. In the examples shown, the fiberexpander 400 includes a multiplexer/demultiplexer 410, 410 a, 410 boptically coupled to the OLT 120, a splitter/combiner 420, 420 a, 420 b,a pair of transmit-EDFAs 430, 430 a, 430 b, a pair of receive-EDFAs 430,430 c, 430 d, and a pair of band multiplexers 440, 440 a, 440 b. Themultiplexer/demultiplexer 410 is optically coupled to the OLT 120 andmay use dense wavelength division multiplexing (DWDM) formultiplexing/demultiplexing optical signals 102. Themultiplexer/demultiplexer 410 includes a multiplexer side (DWDM MUX) 410a for multiplexing downstream optical signals 102 dλ ₁-λ_(n) transmittedfrom the OLT 120 and includes a demultiplexer side (DWDM DEMUX) 410 bfor demultiplexing upstream optical signals 102 u received by the OLT120 from the RN 170. The splitter/combiner 420 includes a splitter side420 a for splitting downstream multiplexed signals 102 dm intorespective first and second split-multiplexed signals 102 dm ₁, 102 dm₂. The splitter/combiner 420 also includes a combiner side 420 b forcombining upstream optical signals 102 ua, 102 ub amplified by thereceive EDFAs 430 c, 430 d into a combined upstream optical signal 102uc input to the demultiplexer 410 b for demultiplexing into upstreamoptical signals 102 uλ ₁-λ_(n) received by the OLT 120.

Referring to FIG. 4A, the OLT 120 a at the first CO 130 a is configuredto transmit the downstream optical signal 102, 102 dλ ₁-λ_(n). In someexamples, the OLT 120 a includes two or more transceivers 122, 122 a-neach configured to transmit a corresponding downstream optical signal102, 102 dλ ₁-λ_(n) at a corresponding target wavelength λ, λ₁-λ_(n). Inother examples, the OLT 120 a includes a laser array configured totransmit multiple optical signals 102, 102 dλ ₁-λ_(n).

In some examples, the multiplexer 410 a (e.g., DWDM MUX) is configuredto multiplex the downlink optical signals 102, 102 dλ ₁-λ_(n) into thedownstream multiplexed signal 102 dm received by the splitter 420 a. Thesplitter 420 a is configured to split the received multiplexed signal102 dm into the first and second split-multiplexed signals 102 dm ₁, 102dm ₂, route the first split-multiplexed signal 102 dm ₁ to the firsttransmit-EDFA 430 a, and route the second split-multiplexed signal 102dm ₂ to the second transmit-EDFA 430 b. When enabled, eachtransmit-EDFAs 430 a, 430 b is configured to amplify the correspondingsplit-multiplexed signal 102 dm ₁, 102 dm ₂ received from the splitter420 a.

The first band multiplexer 440 a is optically coupled the firsttransmit-EDFA 430 a and a first feeder fiber (FF1) 110 a. The firstfeeder fiber 110 a is optically coupled to the first transmit-EDFA 430 aand the first RN 170 a. In the example shown, the first band multiplexer440 a is configured to multiplex between: i) the first optical signal102 a; and ii) the first split-multiplexed signal 102 dm ₁ and a third(upstream) optical signal 102 ua. The first band multiplexer 440 areceives the first split-multiplexed signal 102 dm ₁ from the firsttransmit-EDFA 430 a and passes the first split-multiplexed signal 102 dm₁ as the downstream component 102 da of the first optical signal 102 a.The first band multiplexer 440 a also demultiplexes out the third(upstream) optical signal 102 ua from the first optical signal 102 a andpasses the third (upstream) optical signal 102 ua to the firstreceive-EDFA 430 c. In some examples, the first feeder fiber 110 acorresponds to a default feeder fiber 110, 110 a for the first RN 170 aand is configured to route the first optical signal 102 a between thefirst RN 170 a and the first CO 130 a, which corresponds to the defaultCO 130 a for the first RN 170 a. Since the first optical signal 102 acarries the one or more wavelengths λ₁-λ_(n), the one or morewavelengths λ₁-λ_(n) all pass between the first CO 130 a and the firstRN 170 a.

In other configurations, the first band multiplexer 440 a may include afiltering device configured to allow one or more wavelengths λ₁-λ_(n) ofthe first split-multiplexed signal 102 dm ₁ to pass through the firstfeeder fiber 110 a to the first RN 170 a, while blocking the passagetherethrough of one or more other wavelengths λ₁-λ_(n) of the firstsplit-multiplexed signal 102 dm ₁.

On the other hand, the second band multiplexer 440 b is opticallycoupled to the second transmit-EDFA 430 b and a second feeder fiber(FF2) 110 b. The second feeder fiber 110 b is optically coupled to thesecond transmit-EDFA 430 b and the second RN 170 b. In the exampleshown, the second band multiplexer 440 b is configured to multiplexbetween: i) the second optical signal 102 b; and ii) the secondsplit-multiplexed signal 102 dm ₂ and a fourth (upstream) optical signal102 ub. The second band multiplexer 440 b receives the secondsplit-multiplexed signal 102 dm ₂ from the second transmit-EDFA 430 band passes the second split-multiplexed signal 102 dm ₂ as thedownstream component 102 db of the second optical signal 102 b. Thesecond band multiplexer 440 b also demultiplexes out the fourth(upstream) optical signal 102 ub from the second optical signal 102 band passes the fourth (upstream) optical signal 102 ub to the secondreceive-EDFA 430 d. In some examples, the second feeder fiber 110 bcorresponds to a backup feeder fiber 110, 110 b for the second RN 170 band is configured to route the second optical signal 102 b between thesecond RN 170 b and the first CO 130 a, which corresponds to the backupCO 130 a for the second RN 170 b. Since the second optical signal 102 bcarries the one or more wavelengths λ₁-λ_(n), the one or morewavelengths λ₁-λ_(n) all pass between the first CO 130 a and the secondRN 170 b.

In other configurations, the second band multiplexer 440 b may include afiltering device configured to allow one or more wavelengths λ₁-λ_(n) ofthe second split-multiplexed signal 102 dm ₂ to pass through the secondfeeder fiber 110 b to the second RN 170 b, while blocking the passagetherethrough of one or more other wavelengths λ₁-λ_(n) of the secondsplit-multiplexed signal 102 dm ₂.

In the example shown, the first feeder fiber 110 a optically couplingthe first CO 130 a to the first RN 170 a routes the first optical signal102 a (e.g., a multiplexed signal including the upstream and downstreamoptical signals 102 ua, 102 da) between the first RN 170 a and the firstCO 130 a, which corresponds to the default CO 130 a for the first RN 170a. Likewise, the second feeder fiber 110 b optically coupling the firstCO 130 a to the second RN 170 b routes the second optical signal 102 b(e.g., a multiplexed signal including the upstream and downstreamoptical signals 102 ub, 102 db) between the second RN 170 b and thefirst CO 130 a, which corresponds to the backup CO 130 a for the secondRN 170 b. In some examples, the first optical signal 102 a and thesecond optical signal 102 b are the same signals, and thus carry thesame wavelengths λ₁-λ_(n) and data.

The second CO 130 b includes a substantially identical structure andcomponents as the first CO 130 a. In the examples shown, the second CO130 b optically couples to the second RN 170 b via a third feeder fiber110 c (e.g., Default FF3) and optically couples to the first RN 170 avia a fourth feeder fiber 110 d (e.g., Backup FF4). Here, the second CO130 b may correspond to the default CO 130 b for the second RN 170 b andthe backup CO 130 b for the first RN 170 a. The third feeder fiber 110 coptically coupling the second CO 130 b to the second RN 170 b isconfigured to route a corresponding second optical signal 102 b′ (e.g.,a multiplexed signal which includes upstream and downstream opticalsignals 102 ub′, 102 db′) between the second RN 170 b and the second CO130 b, which corresponds to the default CO 130 b for the second RN 170b. Likewise, the fourth feeder fiber 110 d optically coupling the secondCO 130 b to the first RN 170 a and is configured to route acorresponding first optical signal 102 a′ (e.g., a multiplexed signalwhich includes upstream and downstream optical signals 102 ua′, 102 da′)between the first RN 170 a and the second CO 130 b, which corresponds tothe backup CO 130 b for the first RN 170 a.

In scenarios when the second RN 170 b is not receiving communications(e.g., the second optical signal 102 b′) from the second CO 130 b due toa failure at the second CO 130 b or the corresponding third feeder fiber110 c, the first CO 130 a may backup the second RN 170 b by using thesecond feeder fiber 110 b (Backup FF2) to maintain those communicationswith the second RN 170 b. For instance, the first CO 130 a may transmitthe corresponding second optical signal 102 b along the second feederfiber 110 b to the second RN 170 b. Similarly, in scenarios when thefirst RN 170 a is not receiving communications (e.g., the first opticalsignal 102 a) from the first CO 130 a due to a failure at the first CO130 a or the corresponding first feeder fiber 110 a, the second CO 130 bmay backup the first RN 170 a by using the fourth feeder fiber 110 d(Backup FF4) to maintain those communications with the first RN 170 a.Here, the second CO 130 b may transmit the corresponding first opticalsignal 102 a′ along the fourth feeder fiber 110 d to the first RN 170 a.

In some implementations, the EDFAs 430, 430 a-d are each operablebetween an enabled state and a disabled state. The enabled state of eachEFDA 430 amplifies and allows light (e.g., optical signals) to passthrough the corresponding EFDA 430, while the disabled state of eachEFDA 430 substantially inhibits light from passing through thecorresponding EFDA 430. Thus, disabling an EFDA 430 causes the EFDA 430to operate as an attenuator, and therefore, inhibit almost all receivedlight from passing through the EFDA 430.

With continued reference to FIG. 4A, the enabled state of the firsttransmit-EDFA 430 a is configured to allow the first optical signal 102da transmitted from the OLT 120 a to pass through the firsttransmit-EDFA 430 a to the first RN 170 a. Specifically, the firsttransmit-EDFA 430 a amplifies the first split-multiplexed signal 102 dm₁ in the enabled state and the amplified first split-multiplexed signal102 dm ₁, carrying the one or more wavelengths λ₁-λ_(n), passes throughthe first band multiplexer 440 a to the first RN 170 a. On the otherhand, the disabled state of the first transmit-EDFA 430 a is configuredto substantially inhibit the passing of the first optical signal 102 da(e.g., the first split-multiplexed signal 102 dm ₁) from the OLT 120 athrough the first transmit-EDFA 430 a to the first RN 170 a.

As with the enabled state of the first transmit-EDFA 430 a, the enabledstate of the second transmit-EDFA 430 b is configured to allow thesecond optical signal 102 db transmitted from the OLT 120 a to passthrough the second transmit-EDFA 430 b to the second RN 170 b.Specifically, the second transmit-EDFA 430 b amplifies the secondsplit-multiplexed signal 102 dm ₂ in the enabled state and the amplifiedthe second split-multiplexed signal 102 dm ₂, carrying the one or morewavelengths λ₁-λ_(n), passes through the second band multiplexer 440 bto the second RN 170 b. However, the disabled state of the secondtransmit-EDFA 430 b is configured to substantially inhibit the passingof the second optical signal 102 db (e.g., the second split-multiplexedsignal 102 dm ₂) from the OLT 120 a through the second transmit-EDFA 430b to the second RN 170 b.

In some implementations, the central controller 450 receives a firstremote node-status 452, 452 a (RN1 Status) from the first CO 130 aindicating whether the first RN 170 a is receiving the first opticalsignal 102 a from the first CO 130 a, and receives a second remotenode-status 452, 452 b (RN2 Status) from the second CO 130 b indicatingwhether the second RN 170 b is receiving the second optical signal 102b′ from the second CO 130 b. The central controller 450 may command thefirst CO 130 a (e.g., the default CO 130 a for the first RN 170 a) totransmit the first optical signal 102 a to the first RN 170 a along thefirst fiber feeder 110 a (default FF1) and determine whether the firstRN 170 a receives the first optical signal 102 a from the first CO 130 abased on the first remote node-status 452 a. When the controller 450determines that the first remote node 170 a fails to receive the firstoptical signal 102 a from the first CO 130 a, the controller 450 maycommand the second CO 130 b (e.g., the backup CO 130 b for the first RN170 a) to transmit the corresponding first optical signal 102 a′ to thefirst RN 170 a along the fourth fiber feeder 110 d (e.g., backup FF4).

Similarly, the controller 450 may command the second CO 130 b (e.g., thedefault CO 130 b for the second RN 170 b) to transmit the second opticalsignal 102 b′ to the second RN 170 b along the third fiber feeder 110 c(e.g., default FF3) and determine whether the second RN 170 b receivesthe second optical signal 102 b from the second CO 130 b based on thesecond remote node-status 452 b. When the controller 450 determines thatthe second RN 170 b fails to receive the second optical signal 102 b′from the second CO 130 b, the controller 450 may command the first CO130 a (e.g., the backup CO 130 a for the second RN 170 b) to transmitthe corresponding second optical signal 102 b to the second RN 170 balong the second fiber feeder 110 b (e.g., backup FF2).

The central controller 450 may have supervisory control over each CO 130and may command each CO 130 to transmit first and/or second opticalsignals 102 a, 102 b, 102 a′, 102 b′ to the associated first and/orsecond RNs 170 a, 170 b by commanding at least one of the transmit-EFDAs430, 430 a, 430 b to be in the enabled state. For example, when thecontroller 450 receives the second remote node-status 452, 452 b (e.g.,from the second CO 130 b) indicating that the second RN 170 b is notreceiving communications (e.g., the second optical signal 102 db′) fromthe second CO 130 b, the controller 450 may send EDFA commands 454, 454a to the first CO 130 a that command the second transmit-EFDA 430 b tobe in the enabled state. FIGS. 4A and 4C show the enabled state of thesecond transmit-EFDA 430 b at the first CO 130 a allowing the secondoptical signal 102 db including the one or more wavelengths λ₁-λ_(n)transmitted from the first OLT 120 a to pass through the secondtransmit-EDFA 430 b and along the second feeder fiber 110 b (e.g.,Backup FF2) to the second RN 170 b. Here, the first CO 130 a is servingas a backup to the second RN 170 b to maintain the communicationsthereto that the second CO 130 b is unable to provide due to a failureat the second CO 130 b or along the corresponding third feeder fiber 110c serving the second RN 170 b. Accordingly, enabling the secondtransmit-EFDA 430 b causes the second transmit-EFDA 430 b to amplify thesecond optical signal 102 db (e.g., the second split-demultiplexedsignal 102 dm ₂) so that the first CO 130 a sends the amplified secondoptical signal 102 b along the second feeder fiber 110 b to the secondRN 170 b.

By contrast, when the second remote node-status 452, 452 b indicatesthat the second RN 170 b is receiving communications (e.g., the secondoptical signal 102 b′) from second CO 130 b, the controller 450 may sendEDFA commands 454 a to the first CO 130 a that command the secondtransmit-EDFA 430 b to be in the disabled state. FIG. 4B shows thedisabled state of the second transmit-EDFA 430 b (e.g., “OFF”) at thefirst CO 130 a substantially inhibiting the passing of the secondoptical signal 102 db including the one or more wavelengths λ₁-λ_(n)from the OLT 120 a through the second transmit-EDFA 430 b to the secondRN 170 b. Accordingly, since the second RN 170 b is successfullycommunicating with the associated default second CO 130 b, disabling thesecond transmit-EDFA 430 b at the first CO 130 a prevents the second RN170 b from receiving duplicates of the second optical signal 102 b′, 102db′. Thus, disabling the second transmit-EFDA 430 b ceases theamplification of the second split-multiplexed signal 102 dm ₂ by thesecond transmit-EDFA 430 b, thereby causing the second transmit-EDFA 430b to substantially inhibit the passing of the second optical signal 102db through the second transmit-EDFA 430 b to the second RN 170 b.

The controller 450 may similarly receive the first remote node-status452, 452 a from the first CO 130 a indicating whether the first RN 170 ais receiving communications (e.g., the first optical signal 102 a) fromthe first CO 130 a. In some examples, when the first remote node-status452 a indicates that the first RN 170 a is not receiving the firstoptical signal 102 a from the first CO 130 a, the controller 450 sendsEDFA commands 454, 454 b to the second CO 130 b that command thecorresponding first transmit-EDFA 430 a (not shown) to be in the enabledstate. Here, the enabled state of the first transmit-EDFA 430 a at thesecond CO 130 b allows the corresponding first optical signal 102 a′ totransmit from the second CO 130 b through the fourth feeder fiber 110 d(e.g., Backup FF4) to the first RN 170 a. Accordingly, the second CO 130b serves as a backup to the first RN 170 a to maintain thecommunications thereto that the first CO 130 a is unable to provide dueto a failure at the first CO 130 a or along the corresponding firstfeeder fiber 110 a serving the first RN 170 a. However, when the firstremote node-status 452 a indicates that the first RN 170 a is receivingthe first optical signal 102 a from the first CO 130 a (as shown in theexamples of FIGS. 4A-4C), the controller 450 may send EDFA commands 454b to the second CO 130 b that command the corresponding firsttransmit-EDFA 430 a to be in the disabled state (e.g., “OFF” in FIG.4B), i.e., to prevent the first RN 170 a from receiving duplicates ofthe first optical signal 102 a, 102 da.

Referring to FIG. 4A, the first receive-EDFA 430 c optically couples tothe first OLT 120 a and the first feeder fiber 110 a, while the secondreceive-EDFA 430 d optically couples to the first OLT 120 a and thesecond feeder fiber 110 b. The enabled state of the first receive-EDFA430 c is configured to amplify the third (upstream) optical signal 102ua demultiplexed from the first optical signal 102 a received from thefirst RN 170 a and pass the amplified third optical signal 102 ua to theOLT 120 a. By contrast, the disabled state of the first receive-EDFA 430c is configured to substantially inhibit the passing of the third(upstream) optical signal 102 ua from the first RN 170 a through thefirst receive-EDFA 430 c to the OLT 120 a. Here, the first receive-EDFA430 c attenuates the third (upstream) optical signal 102 ua that passesthrough the first band multiplexer 440 a. In some examples, thecontroller 450 commands the first receive-EDFA 430 c to be in the samestate (i.e., the enabled state or the disabled) as the firsttransmit-EDFA 430 a.

As with the enabled state of the first receive-EDFA 430 c, the enabledstate of the second receive-EDFA 430 d is configured to amplify thefourth (upstream) optical signal 102 ub demultiplexed from the secondoptical signal 102 b received from the second RN 170 b and pass theamplified fourth (upstream) optical signal 102 ub to the OLT 120 a. Onthe other hand, the disabled state of the second receive-EDFA 430 d isconfigured to substantially inhibit the passing of the fourth (upstream)optical signal 102 ub from the second RN 170 b through the secondreceive-EDFA 430 d to the OLT 120 a. Thus, disabling the secondreceive-EDFA 430 d causes the second receive-EDFA 430 d to attenuate thefourth (upstream) optical signal 102 ub that passes through the secondband multiplexer 440 b. In some examples, the controller 450 commandsthe second receive-EDFA 430 d to be in the state (e.g., enabled state orthe disabled state) as the second transmit-EDFA 430 b.

The combiner 420 b receives the amplified third and fourth (upstream)optical signals 102 ua, 102 ub from the first and second receive-EDFAs430 c, 430 d and combines the amplified third and fourth (upstream)optical signals 102 ua, 102 ub into the upstream combined optical signal102 uc. For instance, enabling both of the receive-EDFAs 430 c, 430 dallows the combiner 420 b to combine the third and fourth (upstream)optical signals 102 ua, 102 ub into the upstream combined optical signal102 uc. As used herein, the upstream combined-optical signal 102 uc mayalso be referred to as a combined-multiplexed signal 102 uc. Inscenarios when one of the receive-EDFAs 430 c, 430 d is in the disabledstate, the combiner 420 b only receives the one of the third (upstream)optical signal 102 ua or the fourth (upstream) optical signal 102 ubamplified by the corresponding one of the receive-EDFAs 430 c, 430 d inthe enabled state. Thereafter, the demultiplexer 410 b demultiplexes thecombined-multiplexed signal 102 uc into one or more upstreamdemultiplexed optical signals 102, 102 uλ ₁-λ_(n) and passes the one ormore upstream demultiplexed optical signals 102, 102 uλ ₁-λ_(n) to theOLT 120 a.

As with the first and second transmit-EDFAs 430 a, 430 b, the centralcontroller 450 is in communication with the first and secondreceive-EDFAs 430 c, 430 d, and may command each receive EDFA 430 c, 430d to be in one of the enabled state or the disabled state. For instance,when the controller 450 receives the second remote node-status 452 bindicating that the second RN 170 b is not receiving optical signalsfrom the second CO 130 b, the controller 450 may send the EFDA commands454 a to the first CO 130 a that command the second receive-EFDA 430 dto be in the enabled state. FIGS. 4A and 4C show the enabled state ofthe second receive-EFDA 430 d at the first CO 130 a amplifying thefourth (upstream) optical signal 102 ub received from the second RN 170b along the second feeder fiber 110 b (e.g., Backup FF2) and passing theamplified fourth (upstream) optical signal 102 ub to the first OLT 120a. Likewise, the enabling (default) of the first receive-EDFA 430 cenables amplification by the first receive-EDFA 430 c of the third(upstream) optical signal 102 ua received from the first RN 170 a alongthe first feeder fiber 110 a, and routes the third (upstream) opticalsignal 102 ua to the first OLT 120 a from the first receive-EDFA 430 c.

In some implementations, the central controller 450 receives the secondremote node-status 452 b from the second CO 130 b indicating that thesecond RN 170 b is now receiving the second optical signal 102 b′ fromthe second CO 130 b when the second CO 130 b and/or the correspondingthird feeder fiber 110 c is restored and operational after a failure. Inresponse to the second remote node-status 452 b (i.e., indicating thereceiving of optical signal 102 b′ by the second RN 170 b from thesecond CO 130 b), the controller 450 may send the EFDA commands 454 a tothe first CO 130 a that command the second receive-EDFA 430 d to be inthe disabled state. The disabling the second receive-EDFA 430 d disablesthe amplification by the second receive-EDFA 430 d of the fourth(upstream) optical signal 102 ub received from the second RN 170 b alongthe second feeder fiber 110 b (e.g., Backup FF2). Accordingly, disablingthe amplification by the second receive-EDFA 430 d causes the secondreceive-EDFA 430 d to substantially inhibit receipt of the fourth(upstream) optical signal 102 ub through the second receive-EDFA 430 d.

FIG. 4B shows both the first and second COs 130 a, 130 b and theircorresponding default feeder fibers 110 a, 110 c fully operational suchthat the first CO 130 a communicates with the first RN 170 a along thefirst feeder fiber 110 a and the second CO 130 b communicates with thesecond RN 170 b along the third feeder fiber 110 c. For example, thefirst feeder fiber 110 a (e.g., Default FF1) routes the first opticalsignal 102 a (e.g., a multiplexed signal including the upstream anddownstream optical signals 102 ua, 102 da) between the first CO 130 aand the first RN 170 a. Since the first CO 130 a is only serving thefirst RN 170 a, the one or more wavelengths λ, λ₁-λ_(n) carried by thefirst optical signal 102 a only include temporal information and datafor the first RN 170 a. Likewise, the third feeder fiber 110 c (e.g.,Default FF3) routes the second optical signal 102 b′ (e.g., amultiplexed signal including the upstream and downstream optical signals(102 ub′, 102 db′) between the second CO 130 b and the second RN 170 balong. Since the second CO 130 b is only serving the second RN 170 b,the one or more wavelengths λ, λ₁-λ_(n) carried by the second opticalsignal 102 b′ only include temporal information and data for the secondRN 170 b. Thus, the one or more wavelengths λ, λ₁-λ_(n) associated withthe first optical signal 102 a contain different temporal informationand data than the one or more wavelengths λ, λ₁-λ_(n) associated withthe second optical signal 102 b′.

The controller 450 receives the first remote node-status 452 aindicating that the first RN 170 a is receiving optical signals (e.g.,first optical signal 102 a) from the first CO 130 a, and sends the EDFAcommands 454 b to the second CO 130 b that command both the secondtransmit-EFDA 430 b and the second receive-EFDA 430 d to be in theirrespective disabled state. Accordingly, the second CO 130 b is nottransmitting/receiving any optical signals to or from the first RN 170 aalong the fourth feeder fiber 110 d (e.g., Backup FF4). As with thefirst remote node-status 452 a, the controller 450 also receives thesecond remote node-status 452 b indicating that the second RN 170 b isreceiving optical signals (e.g., second optical signal 102 b′) from thesecond CO 130 b, and sends the EFDA commands 454 a to the first CO 130 athat command both the second transmit-EFDA 430 b and the secondreceive-EFDA 430 d to be in their respective disabled state.Accordingly, the first CO 130 a is not transmitting/receiving anyoptical signals to or from the second RN 170 b along the second feederfiber 110 b (e.g., Backup FF2).

FIG. 4C shows a failure occurring at the second CO 130 b and/or alongthe third feeder fiber 110 c, preventing communication of the secondoptical signal 102 b′ (e.g., a multiplexed signal including the upstreamand downstream optical signals 102 ub′, 102 db′) between the second RN170 b and the second CO 130 b, which corresponds to the default CO 130 bfor the second RN 170 b. The controller 450 receives the second remotenode-status 452 b now indicating that the second RN 170 b is notreceiving optical signals (e.g., second optical signal 102 b′) from thesecond CO 130 b, and sends the EFDA commands 454 a to the first CO 130 athat command both the second transmit-EFDA 430 b and the secondreceive-EFDA 430 d to be in their respective enabled state. Accordingly,the enabled state of the second transmit-EDFA 430 b allows the first CO130 a to backup and offer redundancy to the second RN 170 b bytransmitting the second optical signal 102 b to the second RN 170 balong the second feeder fiber 110 b (e.g., Backup FF2). Likewise, theenabled state of the second receive-EFDA 430 d enables the secondreceive-EFDA 430 d to amplify the fourth (upstream) optical signal 102ub demultiplexed from the second optical signal 102 b received from thesecond RN 170 b and route the amplified fourth optical signal 102 ub tothe first OLT 120 a.

Since the first CO 130 a is now serving both the first RN 170 a and thesecond RN 170 b, the one or more wavelengths λ, λ₁-λ_(n) carried by thefirst optical signal 102 a includes temporal information and data forboth the first RN 170 a and the second RN 170 b. Likewise, the one ormore wavelengths λ, λ₁-λ_(n) carried by the second optical signal 102 bincludes temporal information and data for both the second RN 170 b andthe first RN 170 a. Accordingly, the first optical signal 102 a and thesecond optical signal 102 b are the same signals, and thus carry thesame wavelengths λ₁-λ_(n) and data.

In some examples, the second CO 130 b fails due to an equipment failure,such as a chassis failure of the OLT 120 b, or as a result of a poweroutage at the second CO 130 b. The third feeder fiber 110 c may failwhen the fiber 110 c is cut. For instance, the third feeder fiber 110 cmay be cut during maintenance and then restored when maintenance iscomplete.

Referring to FIG. 5A, in some implementations, the communication system100 includes the first and second COs 130 a, 130 b offering feeder fiberredundancy for multiple RNs 170, 170 a-e to alleviate the possibility ofone of the COs 130 a, 130 b being a point of failure for at least one ofthe RNs 170. The communication system 100 includes multiple feederfibers 110 starting at one CO 130 a and ending at the other CO 130 b toprovide first and second feeder fiber rings (e.g., FF Ring 1 and FF Ring2). The feeder fiber rings enable a single distribution fiber 112 to befed from both COs 130 a, 130 b. Each feeder fiber 110 optically couplesone of the COs 130 to one of the RNs 170 or optically couples two RNs170 together. Each feeder fiber 110 may be “U-Shaped” and each RN 170may include a corresponding optical signal processor 500. Accordingly,the redundant feeder fiber rings serve a large number of customers,allowing the additional costs of implementing redundancy to be shared bya large number of users.

The optical signal processor 500 includes an optical combiner 510, ademultiplexer 160 such as the cyclical AWG 200, and at least one stageof power splitters 180 located after the AWG 200. In the example shown,the optical combiner 510 includes a 2:1 optical combiner (e.g., powersplitter) that optically couples to first and second feeder fibers 110a, 110 b of the first feeder fiber ring (FF Ring 1). The first feederfiber 110 a is optically coupled to the first CO 130 a and the secondfeeder fiber 110 b is optically coupled to the second CO 130 b.Accordingly, the optical combiner 510 enables the AWG 200 to be fed bythe first and second COs 130 a, 130 b.

The AWG 200 receives the optical signal 102 output from the opticalcombiner 510 and performs a wavelength dependent split on the receivedoptical signal 102 to output multiple single-wavelength optical signals104 of each PON 105. For instance, the cyclical AWG 200 separates thedifferent wavelengths λ from the received optical signal 102, and sendseach wavelength PON 105 to each corresponding ONU 140, 140 a-n (FIG. 3).In some examples, the cyclical AWG 200 allows up to 20 differentwavelengths λ, thus giving the ability to stack up to 20 PONs 105. Thepower splitters 180 split the single wavelength optical signals 102 ofthe PONs 105 from the cyclical AWG 200 out to each splitsingle-wavelength optical signal 104, 104 a-n of each PON 105 to eachcorresponding ONU 140. Incorporating splitters 180 after the AWG 200reduces the number of AWGs 200 required and also allows point-to-pointservices to bypass the splitters. While the example optical signalprocessor 500 implements a cyclical AWG 200, the RN 170, 170 a mayimplement other types of demultiplexers.

FIG. 5B shows an example optical signal processor 500 at the RN 170, 170a including a 2:N optical splitter 510 configured to receive the opticalsignals 102 along the first and second feeder fibers 110 a, 110 b andsplit the optical signals 102 into multiple optical signals 104, 104a-n. The configuration of FIG. 5B may be utilized in WDM and TWDMcommunication systems 100 for communicating optical signals between theCO 130 and the ONUs 140 associated with different end users 150, but mayneed optical filters at each ONU 140.

FIG. 5C shows the optical signal processor 500 including the 2:1 opticalcombiner 510 (e.g., 2:1 splitter) configured to combine the opticalsignals along each of the first and second feeder fibers 110 a, 110 binto a combined optical signal 102 input to the cyclical AWG 200. Whilethe 2:1 optical combiner 510 is capable of combining light (e.g.,optical signals 102) from each of the feeder fibers FF1, FF2, the 2:1optical combiner 510 produces about a three (3) decibel (dB) loss oneach port optically coupled to the corresponding feeder fiber FF1, FF2.Greater amplification (e.g., by the transmit-EFDAs 430 a, 430 b) of thetransmitted optical signals 102 is often needed to compensate for thelosses produced by the downstream optical coupler 510. However, for RNs170 implemented over a 50 kilometer (km) passive plant (FIG. 3),increasing the amplification of the optical signals 102 increases costs,consumes more power at the COs 130, and may produce optical signals 102that are not eye-safe.

Referring to FIG. 5D, in some implementations, the optical signalprocessor 500 at the RN 170 uses a two-by-two optical switch 600 inplace of the 2:1 optical combiner of FIG. 5C to alleviate the largelosses (e.g., about 3-dB) associated with 2:1 optical combiners. Thetwo-by-two optical switch 600 is operable to switch between a firststate (FIG. 6A) configured to optically couple the first feeder fiber110 a to the multiplexer/demultiplexer 160, such as the AWG 200, and asecond state (FIG. 6B) configured to optically couple the second feederfiber 110 b to the AWG 200. Here, when one of the feeder fibers 110 a,110 b is optically coupled to the input of the AWG 200, the opticalswitch 600 optically decouples the other feeder fiber 110 a, 110 b fromthe input of the AWG 200. While the optical switch 600 produces lossesless than one (1) dB, the optical switch 600 requires power to switchbetween the first and second states.

In some implementations, the optical switch 600 is optically powered bythe downstream optical signal 102 along the feeder fiber 110 a, 110 bthat is currently optically decoupled from the AWG 200 by the opticalswitch 600. Accordingly, the optical power of the downstream opticalsignal 102 provides the power source for the optical switch 600 totrigger a change from one of the first state or the second state to theother one of the first state or the second state. Accordingly, opticallypowering the optical switch 600 allows the RN 170 to avoid having toincorporate additional controllers for switching the state of theoptical switch 600. Such controllers would otherwise increase powerconsumption at the RNs 170 and would be a potential power failure.

An optical switch control circuit (OSCC) 550 includes at least aphotodiode 520 optically coupled to the two-by-two optical switch 600and a capacitor 540 electrically coupled to the photodiode 520 and thetwo-by-two optical switch 600. The photodiode 520 is configured toreceive light (e.g., optical signal 102) passing through the opticalswitch 600 from one of the feeder fibers 110 a, 110 b and charge thecapacitor 540 to a threshold charge. For instance, the photodiode 520may receive light from a downstream optical signal 102 along the secondfeeder fiber 110 b while the first feeder fiber 110 a is opticallycoupled to the AWG 200. Here, a failure may have occurred at the firstfeeder fiber 110 a or the corresponding first CO 130 a and the second CO130 b is now offering feeder fiber redundancy to the RN 170 bytransmitting optical signals along the second feeder fiber 110 b. Whenthe capacitor 540 is charged to a threshold charge, the capacitor 540triggers the two-by-two optical switch 600 to switch to the other stateto optically couple the other feeder fiber 110 (e.g., the second feederfiber 110 b) to the AWG 200.

FIGS. 6A and 6B show schematic views 600 a, 600 b of the two-by-twooptical switch 600 of FIG. 5D in the first state (FIG. 6A) and thesecond state (FIG. 6B). The optical switch 600 includes a first switchinput 610 optically coupled to a first node input 531 of the RN 170, anda second switch input 612 optically coupled to a second node input 532of the RN 170. The first node input 531 is optically coupled to thefirst feeder fiber 110 a optically coupled to the first CO 130 a, andthe second node input 532 is optically coupled to the second feederfiber 110 b optically coupled to the second CO 130 b. Accordingly, theRN 170 is configured to receive a first optical signal 102 a from thefirst CO 130 a and a second optical signal 102 b from the second CO 130b. In some examples, the first CO 130 a is configured to serve the RN170 as a default and the second CO 130 b is configured to serve the RN170 as a backup in scenarios when a failure occurs at the first CO 130 aor the corresponding first feeder fiber 110 a. Accordingly, the firstand second optical signals 102 a, 102 b may be the same.

The two-by-two optical switch 600 also includes a first switch output620 switchably coupled to the first switch input 610 or the secondswitch input 612, and a second switch output 622 switchably coupled tothe first switch input 610 or the second switch input 612. In theexample shown, the photodiode 520 of the OSCC 550 is optically coupledto the second switch output 622, and the capacitor 540 of the OSCC 550is electrically coupled to the photodiode 520 and the optical switch600.

Referring to FIG. 6A, the first state of the two-by-two optical switch600 includes the first switch input 610 switchably coupled to the firstswitch output 620 and the second switch input 612 switchably coupled tothe second switch output 622. The coupling of the first switch input 610to the first switch output 620 optically couples the first node input531 of the RN 170 to the demultiplexer (DMUX/AWG) 160, 200 to allow theDMUX/AWG 160, 200 to receive the first optical signal 102 a from thefirst CO 130 a.

On the other hand, the coupling of the second switch input 612 to thesecond switch output 622 optically couples the second node input 532 ofthe RN 170 to the photodiode 520 to allow the photodiode 520 to receivethe second optical signal 102 b (if any) from the second CO 130 b. Forinstance, any light received by the second switch input 612 (e.g., fromthe second optical signal 102 b) passes out the second switch output 622to the photodiode 520. The photodiode 520 may use the received light(e.g., the second optical signal 102 b) to provide a charge current 522(e.g., a photocurrent) to charge the capacitor 540. Here, the capacitor540 is associated with an energy storage device from the charge current522. In some examples, the capacitor 540 may be a super capacitor (SC).The optical switch 600 remains in the first state until the capacitor540 is charged to the threshold charge. When the capacitor 540 ischarged to the threshold charge, the capacitor 540 triggers thetwo-by-two optical switch 600 to switch to the second state of FIG. 6B.

FIG. 6B shows the second state of the two-by-two optical switch 600having the first switch input 610 switchably coupled to the secondswitch output 622 and the second switch input 612 switchably coupled tothe first switch output 620. Now, the coupling between the second switchinput 612 and the first switch output 620 optically couples the secondnode input 532 of the RN 170 to the DMUX/AWG 160, 200 to allow theDMUX/AWG 160, 200 to receive the second optical signal 102 b from thesecond CO 130 b.

Conversely, the coupling of the first switch input 610 to the secondswitch output 622 optically couples the first node input 531 of the RN170 to the photodiode 520. Here, any light received by the first switchinput 610 via the first node input 531 passes out the second switchoutput 622 to the photodiode 520. In some examples, the first node input531 is not receiving the first optical signal 102 a along the firstfeeder fiber 110 a due to a failure at the first CO 130 a or along thefirst feeder fiber 110 a. In these examples, the “dashed arrow”indicates that the first optical signal 102 a will pass to thephotodiode 520 once the first CO 130 a or the corresponding first feederfiber 110 a is restored. When the first switch input 610 receives thefirst optical signal 102 a, the first optical signal 102 a will pass outthe second switch output 622 to the photodiode 520. The photodiode 520may use the received light (e.g., the first optical signal 102 a) toprovide the charge current 522 to charge the capacitor 540. Here, theoptical switch 600 remains in the second state until the capacitor 540is charged to the threshold charge. When the capacitor 540 is charged tothe threshold charge, the capacitor 540 is configured to trigger thetwo-by-two optical switch 600 to switch back to the first state of FIG.6A. In some examples, the threshold charge includes a value sufficientlyhigh enough to avoid false triggering. Additionally or alternatively,the OSCC 550 may include a timer that enables the switching betweenstates only after light has been collected for some threshold period oftime. The capacitor 540 may be associated with a boost or driver circuitconfigured to supply sufficient voltage or current for switching thestate of the two-by-two optical switch 600.

FIG. 7 is a schematic view 700 showing operational stages of thetwo-by-two optical switch 600 within a RN 170 during failure andrestoration of the first CO 130 a or corresponding first feeder fiber110 a optically coupled to the RN 170. The OSCC 550 includes thephotodiode 520 optically coupled to the second switch output 622 of theoptical switch 600, and the capacitor 540 electrically coupled to thephotodiode 520 and the optical switch 600. During a first stage (“1”),the first switch input 610 of the optical switch 600 is receiving thefirst optical signal 102 a along the first feeder fiber 110 a from thefirst CO 130 a. The optical switch 600 is in the first state (FIG. 6A)including the first switch input 610 switchably coupled to the firstswitch output 620 to output the first optical signal 102 a to the AWG200 (or other demultiplexer). The “X” denotes that the second switchinput 612 is currently not receiving any optical signals 102 (e.g.,second optical signal 102 b) transmitted by the second CO 130 b alongthe second feeder fiber 110 b. Accordingly, no light is passing to thephotodiode 520 of the OSCC 550 since the second switch input 612 is notcurrently receiving any optical signals.

During a second stage (“2”), the RN 170 ceases receipt of the firstoptical signal 102 a. Specifically, the “X” denotes that the firstswitch input 610 of the optical switch 600 ceases receipt of the firstoptical signal 102 a from the first CO 130 a along the first feederfiber 110 a. Here, the first feeder fiber 110 a has become inactive dueto a failure at the first CO 130 a or the corresponding first feederfiber 110 a. Possible failure scenarios may include fiber cuts to thefirst feeder fiber 110 a, chassis failures of the OLT 120 or otherequipment failures at the first CO 130 a, or loss of power orconnectivity by the first CO 130 a. The controller 450 (FIGS. 4A-4C) mayreceive the remote node-status 452 from the RN 170 indicating that theRN 170 is no longer receiving optical signals 102 (e.g., the firstoptical signal 102 a) from the first CO 130 a, and instructs the secondCO 130 b to transmit the second optical signal 102 b to the RN 170 alongthe second feeder fiber 110 b (which corresponds to the fourth feederfiber 110 d in FIG. 4A). The instructing by the controller 450 mayinclude instructing the controller 450 to send the EFDA instructions 454to the second CO 130 b that instruct the corresponding secondtransmit-EFDA 430 b (FIG. 4A) to be in the enabled state. The enabledstate of the second transmit-EFDA 430 allows the second optical signaltransmitted from the second OLT 120 b of the second CO 130 b to passthrough the second transmit-EFDA 430 b to the RN 170.

However, before the second optical signal 102 b (which corresponds thefirst optical signal 102 a′ from the second CO 130 b of FIG. 4A) isoutput to the AWG 200 for demultiplexing, the optical switch 600 needsto switch to the second state by switchably coupling the second switchinput 612 to the first switch output 620. Here, the optical switch 600uses the optical power from the second optical signal 102 b as the powersource for switching from the first state to the second state. Forinstance, the light associated with the second optical signal 102 breceived by the second switch input 612 passes out to the second switchoutput 622 to the photodiode 520 and the photodiode 520 provides thecharge current 522 to charge the capacitor 540. When the capacitor 540is charged to the charge threshold, the capacitor 540 triggers thetwo-by-two optical switch 600 to switch to the second state.

During a third stage (“3”), the capacitor 540 triggers the opticalswitch 600 to switch to the second state (FIG. 6B) to have the firstswitch input 610 switchably coupled to the second switch output 622 andthe second switch input 612 switchably coupled to the first switchoutput 620. The switching by the capacitor 540 may cause dissipation ofthe capacitor 540. Accordingly, the second state of the optical switch600 allows the second optical signal 102 b transmitted from the secondCO 130 b along the second feeder fiber 110 b to pass out of the firstswitch output 620 and to the AWG 200 for demultiplexing.

During a fourth stage (“4”), the RN 170 ceases receipt of the secondoptical signal 102 b from the second CO 130 b. Specifically, the “X”denotes that the second switch input 612 of the optical switch 600ceases receipt of the second optical signal 102 b from the second CO 130b along the second feeder fiber 110 b. In some examples, the centralcontroller 450 instructs the second CO 130 b to cease transmission ofthe second optical signal 102 b, e.g., by commanding the OLT 120 and/orby sending the EFDA instructions 454 that instruct the secondtransmit-EFDA 430 b to be in the disabled state. The disabled state ofthe second transmit-EFDA 430 b substantially inhibits the passing of thesecond optical signal 102 b transmitted from the OLT 120 b through thesecond transmit-EFDA 430 b to the RN 170. In some implementations, thecentral controller 450 instructs the second CO 130 b to ceasetransmission of the second optical signal 102 b when the controller 450receives the remote node-status 452 indicating that the RN 170 is againreceiving the first optical signal 102 a from the first CO 130 a. Thus,the failure at the first CO 130 a or the corresponding first feederfiber 110 a has been restored, thereby enabling the RN 170 to once againreceive the first optical signal 102 a from the first CO 130 a.

However, before the first optical signal 102 a is output to the AWG 200for demultiplexing, the optical switch 600 needs switch back to thefirst state by switchably coupling the first switch output 620 back tothe first switch output 620. Here, the optical switch 600 uses theoptical power from the first optical signal 102 a as the power sourcefor switching from the second state (FIG. 6B) to the first state (FIG.6A). For instance, the light associated with the first optical signal102 a received by the first switch input 610 passes out to the secondswitch output 622 to the photodiode 520 and the photodiode 520 providesthe charge current 522 to charge the capacitor 540. When the capacitor540 is charged to the charge threshold, the capacitor 540 triggers thetwo-by-two optical switch 600 to switch to the second state.

During a fifth stage (“5”), the capacitor 540 triggers the opticalswitch 600 to switch back to the first state (FIG. 6A) having the firstswitch input 610 switchably coupled to the first switch output 620 andthe second switch input 612 switchably coupled to the second switchoutput 622. The switching by the OSCC 500 may cause dissipation of thecapacitor 540. Accordingly, the first state of the optical switch 600allows the first optical signal 102 a transmitted from the first CO 130a along the first feeder fiber 110 a to pass out of the first switchoutput 620 and to the AWG 200 for demultiplexing.

FIG. 8 provides an example arrangement of operations for a method 800 ofproviding central office (CO) redundancy by allowing multiple COs 130,130 a, 130 b to service a given remote node (RN) 170. At block 802, themethod 800 includes transmitting, from an optical line terminal (OLT)120, 120 a of a first CO 130, 130 a, a first optical signal 102 da. Atblock 804, the method 800 includes amplifying, by a firsttransmit-erbium-doped fiber amplifier (EDFA) 430, 430 a of the first CO130 a, the first optical signal 102 da, and at block 806, sending theamplified first optical signal 102 da from the first CO 130 a along afirst feeder fiber 110, 110 a to a first RN 170, 170 a. At block 808,the method 800 also includes receiving a remote node-status 452indicating whether a second RN 170, 170 b is receiving optical signalsfrom a second CO 130, 130 b. For instance, a central controller 450 incommunication with the first and second COs may receive the remotenode-status 452 from the second CO 130 b.

When the remote-node-status 452 indicates that the second RN 170 b isnot receiving optical signals 102 da from the second CO 130 b, themethod 800 includes, at block 810, transmitting a second optical signal102 db from the OLT of the first CO 130 a. At block 812, the method 800also includes amplifying the second optical signal 102 db by a secondtransmit-EDFA 430, 430 b of the second CO 130 b. Here, the centralcontroller 450 may send EFDA instructions 454 to the secondtransmit-EDFA 430 b that instruct the second transmit-EDFA 430 b to bein the enabled state. At block 814, the method 800 includes sending theamplified second optical signal 102 db from the second CO 130 b along asecond feeder fiber 110, 110 b to the second RN 170 b.

FIG. 9 provides an example arrangement of operations for a method 900 ofswitching a two-by-two optical switch 600 at a remote node (RN) 170 whena backup central office (CO) 130 provides redundancy by servicing the RN170. At block 902, the method 900 includes receiving, at a remote node(RN) 170, a first optical signal 102 a from a first feeder fiber 110,110 a optically coupled to a first CO 130, 130 a. The RN 170 includes afirst node input 531 optically coupled to the first feeder fiber 110 a,a second node input 532 optically coupled to a second feeder fiber 110,110 b optically coupled to a second CO 130, 130 b. The second CO 130 bis configured to transmit a second optical signal 102 b. The first andsecond optical signals 102 a, 102 b may be the same. The two-by-twooptical switch 600 includes a first switch input 610 optically coupledto the first node input 531, a second switch input 612 optically coupledto the second node input 532, a first switch output 620 switchablycoupled to the first switch input 610 or the second switch input 612,and a second switch output 622 switchably coupled to the first switchinput 610 or the second switch input 612. The RN 170 also includes aphotodiode 520 optically coupled to the second switch output 622 and acapacitor 540 electrically coupled to the photodiode 520 and thetwo-by-two optical switch 600.

At block 904, the method 900 includes outputting the first opticalsignal 102 a from the first switch output 620 of the RN 170. Here, thefirst optical signal 102 a serves as an input demultiplexed by the AWG200 into multiple demultiplexed-optical signals 104, 104 a-n. At block906, the method 900 also includes ceasing receipt of the first opticalsignal 102 a at the RN 170. For instance, the RN 170 may cease receiptof the first optical signal 102 a when the first CO 130 a orcorresponding first feeder fiber 110 a fails. At block 908, the method900 includes receiving, at the second node input 532 of the RN 170, thesecond optical signal from the second feeder fiber 110 b opticallycoupled to the second CO 130 b. Here, the second optical signal 102 b ispassing out of the second switch output 622 to the photodiode 520.

At block 910, the method 900 also includes charging, by the photodiode520, the capacitor 540 to the threshold charge. At block 912, the method900 includes switching, by the capacitor 540, the two-by-two opticalswitch 600 to have the first switch input 610 switchably coupled to thesecond switch output 622 and the second switch input 612 switchablycoupled to the first switch output 620. Here, the two-by-two opticalswitch is in the second state of FIG. 6B, thereby allowing the secondoptical signal 102 b to pass out of the first switch output 620 fordemultiplexing by the AWG 200. The switching by the capacitor 540 causesdissipation of the capacitor 540.

FIG. 10 is a schematic view of an example computing device 1000 that maybe used to implement the systems and methods described in this document.The computing device 1000 is intended to represent various forms ofdigital computers, such as laptops, desktops, workstations, personaldigital assistants, servers, blade servers, mainframes, and otherappropriate computers. The components shown here, their connections andrelationships, and their functions, are meant to be exemplary only, andare not meant to limit implementations of the inventions describedand/or claimed in this document.

The computing device 1000 includes a processor 1010, memory 1020, astorage device 1030, a high-speed interface/controller 1040 connectingto the memory 1020 and high-speed expansion ports 1050, and a low speedinterface/controller 1060 connecting to low speed bus 1070 and storagedevice 1030. Each of the components 1010, 1020, 1030, 1040, 1050, and1060, are interconnected using various busses, and may be mounted on acommon motherboard or in other manners as appropriate. The processor1010 can process instructions for execution within the computing device1000, including instructions stored in the memory 1020 or on the storagedevice 1030 to display graphical information for a graphical userinterface (GUI) on an external input/output device, such as display 1080coupled to high speed interface 1040. In other implementations, multipleprocessors and/or multiple buses may be used, as appropriate, along withmultiple memories and types of memory. Also, multiple computing devices1000 may be connected, with each device providing portions of thenecessary operations (e.g., as a server bank, a group of blade servers,or a multi-processor system).

The memory 1020 stores information non-transitorily within the computingdevice 1000. The memory 1020 may be a computer-readable medium, avolatile memory unit(s), or non-volatile memory unit(s). Thenon-transitory memory 1020 may be physical devices used to storeprograms (e.g., sequences of instructions) or data (e.g., program stateinformation) on a temporary or permanent basis for use by the computingdevice 1000. Examples of non-volatile memory include, but are notlimited to, flash memory and read-only memory (ROM)/programmableread-only memory (PROM)/erasable programmable read-only memory(EPROM)/electronically erasable programmable read-only memory (EEPROM)(e.g., typically used for firmware, such as boot programs). Examples ofvolatile memory include, but are not limited to, random access memory(RAM), dynamic random access memory (DRAM), static random access memory(SRAM), phase change memory (PCM) as well as disks or tapes.

The storage device 1030 is capable of providing mass storage for thecomputing device 1000. In some implementations, the storage device 1030is a computer-readable medium. In various different implementations, thestorage device 1030 may be a floppy disk device, a hard disk device, anoptical disk device, or a tape device, a flash memory or other similarsolid state memory device, or an array of devices, including devices ina storage area network or other configurations. In additionalimplementations, a computer program product is tangibly embodied in aninformation carrier. The computer program product contains instructionsthat, when executed, perform one or more methods, such as thosedescribed above. The information carrier is a computer- ormachine-readable medium, such as the memory 1020, the storage device1030, or memory on processor 1010.

The high speed controller 1040 manages bandwidth-intensive operationsfor the computing device 1000, while the low speed controller 1060manages lower bandwidth-intensive operations. Such allocation of dutiesis exemplary only. In some implementations, the high-speed controller1040 is coupled to the memory 1020, the display 1080 (e.g., through agraphics processor or accelerator), and to the high-speed expansionports 1050, which may accept various expansion cards (not shown). Insome implementations, the low-speed controller 1060 is coupled to thestorage device 1030 and low-speed expansion port 1070. The low-speedexpansion port 1070, which may include various communication ports(e.g., USB, Bluetooth, Ethernet, wireless Ethernet), may be coupled toone or more input/output devices, such as a keyboard, a pointing device,a scanner, or a networking device such as a switch or router, e.g.,through a network adapter.

The computing device 1000 may be implemented in a number of differentforms, as shown in the figure. For example, it may be implemented as astandard server 1000 a or multiple times in a group of such servers 1000a, as a laptop computer 1000 b, or as part of a rack server system 1000c.

Various implementations of the systems and techniques described hereincan be realized in digital electronic and/or optical circuitry,integrated circuitry, specially designed ASICs (application specificintegrated circuits), computer hardware, firmware, software, and/orcombinations thereof. These various implementations can includeimplementation in one or more computer programs that are executableand/or interpretable on a programmable system including at least oneprogrammable processor, which may be special or general purpose, coupledto receive data and instructions from, and to transmit data andinstructions to, a storage system, at least one input device, and atleast one output device.

These computer programs (also known as programs, software, softwareapplications or code) include machine instructions for a programmableprocessor, and can be implemented in a high-level procedural and/orobject-oriented programming language, and/or in assembly/machinelanguage. As used herein, the terms “machine-readable medium” and“computer-readable medium” refer to any computer program product,non-transitory computer readable medium, apparatus and/or device (e.g.,magnetic discs, optical disks, memory, Programmable Logic Devices(PLDs)) used to provide machine instructions and/or data to aprogrammable processor, including a machine-readable medium thatreceives machine instructions as a machine-readable signal. The term“machine-readable signal” refers to any signal used to provide machineinstructions and/or data to a programmable processor.

The processes and logic flows described in this specification can beperformed by one or more programmable processors executing one or morecomputer programs to perform functions by operating on input data andgenerating output. The processes and logic flows can also be performedby special purpose logic circuitry, e.g., an FPGA (field programmablegate array) or an ASIC (application specific integrated circuit).Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor will receive instructions and data from a read only memory ora random access memory or both. The essential elements of a computer area processor for performing instructions and one or more memory devicesfor storing instructions and data. Generally, a computer will alsoinclude, or be operatively coupled to receive data from or transfer datato, or both, one or more mass storage devices for storing data, e.g.,magnetic, magneto optical disks, or optical disks. However, a computerneed not have such devices. Computer readable media suitable for storingcomputer program instructions and data include all forms of non-volatilememory, media and memory devices, including by way of examplesemiconductor memory devices, e.g., EPROM, EEPROM, and flash memorydevices; magnetic disks, e.g., internal hard disks or removable disks;magneto optical disks; and CD ROM and DVD-ROM disks. The processor andthe memory can be supplemented by, or incorporated in, special purposelogic circuitry.

To provide for interaction with a user, one or more aspects of thedisclosure can be implemented on a computer having a display device,e.g., a CRT (cathode ray tube), LCD (liquid crystal display) monitor, ortouch screen for displaying information to the user and optionally akeyboard and a pointing device, e.g., a mouse or a trackball, by whichthe user can provide input to the computer. Other kinds of devices canbe used to provide interaction with a user as well; for example,feedback provided to the user can be any form of sensory feedback, e.g.,visual feedback, auditory feedback, or tactile feedback; and input fromthe user can be received in any form, including acoustic, speech, ortactile input. In addition, a computer can interact with a user bysending documents to and receiving documents from a device that is usedby the user; for example, by sending web pages to a web browser on auser's client device in response to requests received from the webbrowser.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the disclosure. Accordingly, otherimplementations are within the scope of the following claims.

What is claimed is:
 1. A carrier office comprising: an optical lineterminal (OLT) configured to transmit first and second optical signals;a first transmit-erbium-doped fiber amplifier (EDFA) optically coupledto the OLT and a first feeder fiber, the first feeder fiber opticallycoupled to a first remote node (RN), the first transmit-EDFA operablebetween a respective enabled state and a respective disabled state, theenabled state of the first transmit-EDFA configured to allow the firstoptical signal transmitted from the OLT to pass through the firsttransmit-EDFA to the first RN, the disabled state of the firsttransmit-EDFA configured to substantially inhibit the passing of thefirst optical signal from the OLT through the first transmit-EDFA to thefirst RN; a first receive-EDFA optically coupled to the OLT and thefirst feeder fiber, the first receive-EDFA operable between a respectiveenabled state and a respective disabled state, the enabled state of thefirst receive-EDFA configured to allow a third optical signal from thefirst RN to pass through the first receive-EDFA to the OLT, the disabledstate of the first receive-EDFA configured to substantially inhibit thepassing of the third optical signal from the first RN through the firstreceive-EDFA to the OLT; a second transmit-EDFA optically coupled to theOLT and a second feeder fiber, the second feeder fiber optically coupledto a second RN, the second transmit-EDFA operable between a respectiveenabled state and a respective disabled state, the enabled state of thesecond transmit-EDFA configured to allow the second optical signaltransmitted from the OLT to pass through the second transmit-EDFA to thesecond RN, the disabled state of the second transmit-EDFA configured tosubstantially inhibit the passing of the second optical signal from theOLT through the second transmit-EDFA to the second RN; and a secondreceive-EDFA optically coupled to the OLT and the second feeder fiber,the second receive-EDFA operable between a respective enabled state anda respective disabled state, the enabled state of the secondreceive-EDFA configured to allow a fourth optical signal from the secondRN to pass through the second receive-EDFA to the OLT, the disabledstate of the second receive-EDFA configured to substantially inhibit thepassing of the fourth optical signal from the second RN through thesecond receive-EDFA to the OLT.
 2. The carrier office of claim 1,wherein the OLT comprises a laser array configured to transmit multipleoptical signals.
 3. The carrier office of claim 1, wherein the OLTcomprises two or more transceivers each configured to transmit acorresponding downstream optical signal at a corresponding targetwavelength.
 4. The carrier office of claim 1, further comprising: amultiplexer optically coupled to the OLT and configured to multiplex thefirst and second optical signals into a multiplexed signal; and asplitter optically coupled to the multiplexer and the first and secondtransmit-EDFAs, the splitter configured to: split the multiplexed signalreceived from the multiplexer into first and second split-multiplexedsignals; route the first split-multiplexed signal to the firsttransmit-EDFA; and route the second split-multiplexed signal to thesecond transmit-EDFA.
 5. The carrier office of claim 4, furthercomprising: a first band multiplexer optically coupled to the firsttransmit-EDFA and the first feeder fiber, the first band multiplexerconfigured to receive the first split-multiplexed signal from the firsttransmit-EDFA and pass the first split-multiplexed signal as adownstream component of the first optical signal to the first RN; and asecond band multiplexer optically coupled to the second transmit-EDFAand the second feeder fiber, the second band multiplexer configured toreceive the second split-multiplexed signal from the secondtransmit-EDFA and pass the second split-multiplexed signal as adownstream component of the second optical signal to the second RN. 6.The carrier office of claim 1, further comprising a controller incommunication with the first and second transmit-EDFAs, wherein thecontroller is configured to: receive a remote node status indicatingwhether the second RN is receiving optical signals from another carrieroffice; and when the remote node status indicates that the second RN isnot receiving optical signals from the other carrier office, instructingthe second transmit-EDFA to be in the enabled state.
 7. The carrieroffice of claim 6, wherein the controller is further configured to, whenthe remote node status indicates that the second RN is receiving opticalsignals from the other carrier office, instructing the secondtransmit-EDFA to be in the disabled state.
 8. The carrier office ofclaim 1, further comprising: a first band multiplexer optically coupledto the first receive-EDFA and the first feeder fiber, the first bandmultiplexer configured to demultiplex out the third optical signal fromthe first optical signal and pass the third optical signal to the firstreceive-EDFA; and a second band multiplexer optically coupled to thesecond receive-EDFA and the second feeder fiber, the second bandmultiplexer configured to demultiplex out the fourth optical signal fromthe second optical signal and pass the fourth optical signal to thesecond receive-EDFA.
 9. The carrier office of claim 8, furthercomprising: a combiner optically coupled to the first and secondreceive-EDFAs, the combiner configured to: receive the third and fourthoptical signals from the first and second receive-EDFAs; and combine thethird and fourth optical signals into a combined-multiplexed signal; anda demultiplexer optically coupled to the combiner and the OLT, thedemultiplexer configured to: demultiplex the combined-multiplexed signalinto first and second demultiplexed optical signals; and route the firstand second demultiplexed optical signals to the OLT.
 10. A methodcomprising: transmitting a first optical signal from a first carrieroffice (CO) along a first feeder fiber to a first remote node (RN), thefirst CO comprising: an optical line terminal (OLT) configured totransmit the first optical signal and a second optical signal; a firsttransmit-erbium-doped fiber amplifier (EDFA) optically coupled to theOLT and the first feeder fiber, the first feeder fiber optically coupledto the first RN, the first transmit-EDFA operable between a respectiveenabled state and a respective disabled state, the enabled state of thefirst transmit-EDFA configured to allow the first optical signaltransmitted from the OLT to pass through the first transmit-EDFA to thefirst RN, the disabled state of the first transmit-EDFA configured tosubstantially inhibit the passing of the first optical signal from theOLT through the first transmit-EDFA to the first RN; a firstreceive-EDFA optically coupled to the OLT and the first feeder fiber,the first receive-EDFA operable between a respective enabled state and arespective disabled state, the enabled state of the first receive-EDFAconfigured to allow a third optical signal from the first RN to passthrough the first receive-EDFA to the OLT, the disabled state of thefirst receive-EDFA configured to substantially inhibit the passing ofthe third optical signal from the first RN through the firstreceive-EDFA to the OLT; a second transmit-EDFA optically coupled to theOLT and a second feeder fiber, the second feeder fiber optically coupledto a second RN, the second transmit-EDFA operable between a respectiveenabled state and a respective disabled state, the enabled state of thesecond transmit-EDFA configured to allow the second optical signaltransmitted from the OLT to pass through the second transmit-EDFA to thesecond RN, the disabled state of the second transmit-EDFA configured tosubstantially inhibit the passing of the second optical signal from theOLT through the second transmit-EDFA to the second RN; and a secondreceive-EDFA optically coupled to the OLT and the second feeder fiber,the second receive-EDFA operable between a respective enabled state anda respective disabled state, the enabled state of the secondreceive-EDFA configured to allow a fourth optical signal from the secondRN to pass through the second receive-EDFA to the OLT, the disabledstate of the second receive-EDFA configured to substantially inhibit thepassing of the fourth optical signal from the second RN through thesecond receive-EDFA to the OLT; receiving a remote node statusindicating whether the second RN is receiving optical signals from asecond CO; and when the remote node status indicates that the second RNis not receiving optical signals from the second CO, instructing thesecond transmit-EDFA to be in the enabled state.
 11. The method of claim10, further comprising, when the remote node status indicates that thesecond RN is receiving optical signals from the second CO, instructingthe second transmit-EDFA to be in the disabled state.
 12. The method ofclaim 10, wherein the OLT comprises a laser array configured to transmitmultiple optical signals.
 13. The method of claim 10, wherein the OLTcomprises two or more transceivers each configured to transmit acorresponding downstream optical signal at a corresponding targetwavelength.
 14. The method of claim 10, wherein the first CO furthercomprises: a multiplexer optically coupled to the OLT and configured tomultiplex the first and second optical signals into a multiplexedsignal; a splitter optically coupled to the multiplexer and the firstand second transmit-EDFAs, the splitter configured to: split themultiplexed signal received from the multiplexer into first and secondsplit-multiplexed signals; route the first split-multiplexed signal tothe first transmit-EDFA; and route the second split-multiplexed signalto the second transmit-EDFA; a first band multiplexer optically coupledto the first transmit-EDFA and the first feeder fiber, the first bandmultiplexer configured to receive the first split-multiplexed signalfrom the first transmit-EDFA and pass the first split-multiplexed signalas a downstream component of the first optical signal to the first RN;and a second band multiplexer optically coupled to the secondtransmit-EDFA and the second feeder fiber, the second band multiplexerconfigured to receive the second split-multiplexed signal from thesecond transmit-EDFA and pass the second split-multiplexed signal as adownstream component of the second optical signal to the second RN. 15.The method of claim 10, wherein the first CO further comprises: a firstband multiplexer optically coupled to the first receive-EDFA and thefirst feeder fiber, the first band multiplexer configured to demultiplexout the third optical signal from the first optical signal and pass thethird optical signal to the first receive-EDFA; and a second bandmultiplexer optically coupled to the second receive-EDFA and the secondfeeder fiber, the second band multiplexer configured to demultiplex outthe fourth optical signal from the second optical signal and pass thefourth optical signal to the second receive-EDFA.
 16. The method ofclaim 15, wherein the first CO further comprises: a combiner opticallycoupled to the first and second receive-EDFAs, the combiner configuredto: receive the third and fourth optical signals from the first andsecond receive-EDFAs; and combine the third and fourth optical signalsinto a combined-multiplexed signal; and a demultiplexer opticallycoupled to the combiner and the OLT, the demultiplexer configured to:demultiplex the combined-multiplexed signal into first and seconddemultiplexed optical signals; and route the first and seconddemultiplexed optical signals to the OLT.